Heating 29 gallons is going to take a long time no doubt. But if you just put 5 or 10 at a time in there it should be fine. I fill mostly 5 gallon hot packed pails, I do have a 15 gallon stainless barrel also. When I want to pack syrup I use my propane bottler with steam pan, it heats 5 gallons in about a half hour with 50° syrup, might take 45 or more if your syrup is 25°.
If I fill it full it takes much longer, also of note once you get it up to temperature it heats the next batch up much faster.
I have all intentions of replacing my Small canner with a water Jacketed electric one, even if it takes 2 hours to heat 25 gallons I can plan for it and you dont have to worry about it going over temp like you do with propane so in theory you could start heating and go do something else for awhile.

Besides canning up 25 gallons would be a once a month thing on average.